The extensive wiring already undertaken to provide cable television service via cable networks and the large bandwidth, relative to that available on competing connections make it an attractive medium for providing access to services such as the Internet. The Internet, a world-wide-network of interconnected computers, provides multi-media content including audio, video, graphics and text that often require a large bandwidth for downloading and viewing.
Cable systems, as implemented today, have a tree structure. There is a "head-end" connected by branches to downstream nodes. A node may have a cable modem or other equipment to boost the signal strength, split the signal and other functions. These nodes are, in turn, connected by further branches to other nodes, with the tree finally terminating at customer premises. Coaxial or fiber-optic cables are used for branches although other connections, including wireless connections are possible. Ordinarily, there are no "closed loops" in such a system. In other words, there is typically only one functional downstream path from the head-end to any given node. Customer premise equipment communicates with the head-end of the cable system either by a return path outside of the cable system, or, in newer systems, with bandwidth from the cable network set aside for a return path.
Most Internet Service Providers allow customers to connect to the Internet via a telephone line connection to the Public Switched Telephone Network. The available data rates, usually less than 56,000 bps, are much slower than the about 10 Mbps to 30+ Mbps available on a coaxial cable or Hybrid Fiber/Coaxial cable network.
However, most cable television networks have installed uni-directional cable networks, supporting only a "downstream" data path. A downstream data path is the flow of data from a cable system head-end to a customer. Typically, a return, or "upstream," data path is via a telephone network (i.e., a "telephony return"). An upstream data path facilitates the flow of data to the cable system head-end. Upstream path also includes the flow of data to a device that sends data to the cable modem on a down-stream path. A cable system with an upstream connection to a telephony network is called a "data-over-cable system with telephony return." Some of the cable systems provide two-way service allowing use of some of the bandwidth in a cable network for upstream data paths.
An exemplary data-over-cable system includes a cable modem, a cable-modem-termination system at the head-end, a cable television network, equipment at the nodes for signal processing, and, possibly, a telephony-remote-access-concentrator to manage an upstream telephony path via the public-switched-telephone-network. The cable-modem-termination system and the telephony-remote-access-concentrator together comprise a "telephony-return-termination system."
A data-over-cable system is typically connected to other devices and networks such as the Internet. Most of the connections to other networks are made at the head-end. When telephony return is present, it is possible to address upstream traffic via the telephony-remote-access-concentrator to the Internet without having to go through the cable-modem-termination system. On the other hand, in a two-way data-over-cable system, in the absence of additional upstream paths the outgoing and incoming data go through the cable-modem-termination system.
The cable-modem-termination system, at the head-end, receives data-packets and transmits them downstream via the cable network to a cable modem, which may in turn send them on to customer premise equipment or further downstream. The customer premise equipment may respond by sending data-packets to the cable modem, which, in turn, sends the data-packets upstream.
When a cable modem in a data-over-cable system is initialized, at least one downstream path, typically 6 MHz wide, from the cable-modem-termination system to the cable modem is set up. Six MHz is also the typical bandwidth for a television channel. The allocation is made so as to ensure coexistence of television broadcasts and data connections. In addition, at least one upstream path, either within the cable network, or via an external connection like the public switched telephone network, is established. The upstream path, even when within the cable network, does not usually have a bandwidth of 6 MHz. The occupied frequency ranges are different as well. Downstream paths are typically in the range of 50 MHz to approximately 1 GHz while the upstream paths are within 5 MHz to 42 MHz with a specific slot defined by the cable-modem-termination system.
As a cable modem is initialized in a data-over-cable system, it registers with a cable-modem-termination system. As part of a registration request message, the cable modem forwards configuration information to the cable-modem-termination system. This exchange also establishes the properties of the cable modem to the cable-modem-termination system. If the data-over-cable system supports Quality-of-Service, data-over-cable system may allocate resources to the cable modem in the course of registration.
Configuration information forwarded to a cable-modem-termination system from a cable modem is accompanied by Class-of-Service and Quality-of-Service and other parameters. As is known in the art, Class-of-Service provides a reliable transport facility independent of the Quality-of-Service. Class-of-Service parameters include maximum downstream data rates, maximum upstream data rates, upstream channel priority, guaranteed minimum data rates, guaranteed maximum data rate and other parameters. Quality-of-Service collectively specifies the performance of a network service that a device expects on a network. Quality-of-Service parameters include transit delay expected to deliver data to a specific destination, the level of protection from unauthorized monitoring or modification of data, cost for delivery of data, expected residual error probability, the relative priority associated with the data and other parameters.
A cable-modem-termination system, at the head-end of the data-over-cable system, usually does not address the entire bandwidth, from approximately 50 MHz to about 1 GHz, available in the cable network. A cable-modem-termination system designed to address the entire bandwidth available in the cable network is typically not cost effective. This limitation is handled, in part, by utilizing more than one cable-modem-termination system at the head-end such that each cable-modem-termination system addresses only a fraction of the possible bandwidth. This is possible because many of the tasks can be performed in parallel without extensive cross-communications between the multiple cable-modem-termination systems.
Each cable-modem-termination system is an expensive asset. Hence, optimal utilization of its capacity is warranted. In light of the above, adding another cable-modem-termination system is not always the preferred option when faced with increased demands on the system. For instance, users may be allowed to access system resources only after registering and receiving approval for their requested use of system resources. This permits resource allocation based on satisfying the needs of users allowed access to the data-over-cable system and avoid a freezing up the entire system. At the same time system resources do not have to match peak demand while being idle most of the time.
Furthermore, an entire 6 MHz bandwidth of a downstream path is not exclusively used by a single cable modem. Efficient use of such a large bandwidth requires sending data to more than one cable modem by addressing data-packets to individual cable modems listening on the same downstream path. A data-over-cable system also permits addressing of data-packets to more than one cable modem by means of broadcast addressing. Despite these built in strategies to promote efficient use of data-over-cable system resources, cable-modem-termination system resources are not typically used efficiently in a data-over-cable system.